The present invention is related to a scanning CRT (cathode ray tube) display system in which linearity compensation is provided for an electron beam deflection yoke. More specifically, the present invention is related to providing linearity compensation for a display system horizontal deflection yoke.
In scanning CRT display systems it is known that the various prior techniques utilized to control the horizontal deflection yoke current can result in nonlinear control of the scanning electron beam whose horizontal position is determined by the deflection yoke. During a trace cycle the electron beam sweeps across a CRT surface and defines one horizontal line of a visual frame, and during a retrace cycle the beam is repositioned so as to define the next horizontal line during the next trace cycle. Typically during the horizontal trace cycle of a scanning CRT display system it is desired to provide a repeatable and substantially linear variation of the horizontal deflection yoke current. However, in practice various linearity control circuits and devices must be utilized to insure this repeatability and linear variation. One way which is typically used to achieve such linearity is the use of a fixed inductance linearity coil, wound on a fixed magnetic core, connected in series with the horizontal deflection yoke. This linearity coil is constructed and poled such that its magnetic core saturates at a predetermined amount of deflection yoke current so as to effectively linearize the rate of increase of deflection yoke current during the trace cycle. This type of linearity compensation is suitable in some applications where a bipolar transistor or a tube is used to control the deflection yoke current. However, if high frequency horizontal scan rates, such as 64 kHz, are to be implemented, bipolar transistors and tubes either cannot implement these scan rates or may not provide a cost effective way of achieving these scan rates with low drive power.
In order to achieve high frequency scan rates, which are desired in some display applications, it may be necessary to utilize high power FET (Field Effect Transistor) transistors which can implement the more rapid required switching of the deflection yoke current. However, it has been found that these high power, high frequency FET devices typically have undesirable operating characteristics which prevent their use in controlling the horizontal deflection yoke current. One of these undesirable characteristics is that the on resistance between the output drain and source terminals of the FET device is extremely temperature dependent. This therefore results in substantial linearity variations for the horizontal deflection current from the time the FET device is first turned on until the time that the FET device arrives at its final temperature. The use of the conventional fixed inductance, fixed magnetic core linearity coil cannot compensate for these temperature variable operating characteristics of the high frequency FET device. For prior low frequency drive devices which were used to control the horizontal deflection current, temperature variation of the output characteristics of these drive devices was not a problem. Thus the prior art did not recognize the existence of any linearity problem which required compensation by anything other than a fixed inductance linearity coil wound on a magnetic core with the current in the linearity coil solely determining the saturation of its magnetic core. The prior art also did not recognize the existence of any problem related to the temperature variation of the output characteristics of the drive device which is used to control the horizontal deflection yoke current.